£100 for each monitor is extremely hard to do. Tripple Monitor Setups need a GPU that can handle 3 outputs at the same time. Most of the GPUs do this by using 2 Display Port and 1 HDMI output or a different combination of DP/HDMI and DVI. The Problem is that cheap Monitors often come without display port connector and so you would have to use adapters that add to the overall price. I would not go smaller than 24" for any productive usage. Otherwise it's more recommended to just use 1 or 2 bigger monitors than a tripple setup. For the model you have no big selection you can choose from. Just look what each manufacturer has to offer in the extremely low price class and live with it. With £300 I would go for a nice 27" 1440p single monitor instead of 3 extremely cheap garbage monitors.